Because of the lattice mismatch between a germanium lattice and a silicon lattice, epitaxial growth of germanium directly on a silicon wafer results in a high density of dislocations at the germanium/silicon interface. These dislocations adversely affect the electrical performance of germanium devices so fabricated. In order to reduce the dislocation at the germanium to silicon interface, a germanium epitaxial lateral overgrowth (ELO) technique has been developed.
Li et al. demonstrated that germanium may be selectively grown on silicon (100) through openings in a SiO2 nanotemplate by solid source molecular beam epitaxy (MBE). Germanium islands grow in the template windows and coalesce on top of the template, forming an ELO layer. Cross sectional TEM images show that the germanium seeds and the ELO layer are free of threading dislocations. Stacking faults are generated, but terminate within 70 nm of the germanium-silicon interface, while twin-plane defects along {111} planes on either side of the germanium seed are observed in the ELO layer. Li et al., Selective growth of germanium on Si (100) through vias of SiO2 nanoplate using solid source molecular beam epitaxy, Applied Physics Letters, Vol. 83, No. 24, pp 5032-5034 (2003); and Li et al., Heteroepitaxy of high-quality Ge on Si by nanoscale Ge seeds grown through a thin layer of SiO2, Applied Physics Letters, Vol. 85, No. 11, pp 1928-1930 (2004).
Langdo et al. described selectively grown germanium on a SiO2/Si substrate in 100 nm holes by chemical vapor deposition of high quality at the top surface, compared to conventional germanium lattice-mismatched grown on planar silicon substrates. The threading dislocation generated at the germanium/silicon interface are blocked at the oxide sidewall by the epitaxial necking mechanism. Defects at the germanium film surface only arise at the merging of epitaxial lateral overgrowth fronts from neighboring holes, Langdo et al., High quality Ge on Si by epitaxial necking, Applied Physics Letters, Vol. 76, No. 25, pp 3700-3702 (2000).
As shown in FIG. 1, a cross-section diagram of a germanium thin film 10 fabricated according to prior art processes on a silicon substrate 12 having a SiO2 layer 14 thereon, exhibits epitaxial necking 16 having zero threading dislocations at the germanium film surface, however, twin-plane defects 18 are generated because of the merging of epitaxial lateral overgrowth fronts from neighboring holes. Although films may be fabricated using different growth methods, e.g., molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), films fabricated by either method have a dislocation free epitaxial lateral overgrowth layer, but exhibit twin-plane defects, which are generated in the ELO layer because of the merging of ELO fronts from neighboring holes. Electrical characteristics of the germanium devices fabricated on an ELO-formed germanium layer do not exhibit electrical properties any better than devices fabricated on directly-grown germanium layers on silicon wafers. This is because that the twin-plane defects are a crystal defect that, electrically, behaves similar to a dislocation.